Optical isolators are essential elements in many optical systems for protecting a light source, such as a laser, from being exposed to light which is reflected back at the light source. Such reflected light, known as “optical feedback,” may cause the light source to become unstable or may even damage the light source. The problem is especially difficult in optical systems employing lasers that emit a relatively high output beam power where even surfaces of transmissive optical elements, or relatively small discontinuities or mismatches in optical waveguides can produce sufficient reflections to give rise to deleterious optical feedback.
It is known to incorporate an optical isolator in the path of the laser output beam, near the laser cavity exit aperture, to isolate the laser from reflected laser light and thereby avoid or reduce optical feedback. An optical isolator permits the forward transmission of a radiation beam, in this case the laser output beam, while simultaneously preventing the reverse transmission of the same radiation beam, with a high degree of extinction. Thus, the laser energy reflected back towards the laser from various sources of reflection is trapped, extinguished or reflected by the optical isolator.
Optical isolators based on the Faraday polarization rotation effect are available for use in laser systems. Such a conventional optical isolator is illustrated in FIG. 1. The conventional optical isolator 50 includes, a first polarizer 58 for linearly polarizing a light wave in a first direction 62 and a second polarizer 60 for linearly polarizing a light wave in a second direction 64, a longitudinal magnet 52 surrounding a magneto-optical medium 54, which may be in the form of an optical waveguide, for example. The magnet 52 applies a longitudinal magnetic field 56 to the magneto-optical medium 54.
In operation, an incident light wave is polarized by first polarizer 58 in a first direction 62. If the incident light wave is plane polarized, the first direction 62 of polarization should coincide with the polarization of the incident light wave as it leaves the light source. This polarized light wave then enters the magneto-optical medium 54, where a permanent magnet 52, or alternatively an electromagnet, applies a magnetic field 56 that causes a rotation of the plane of polarization of the light wave by 45 degrees, as shown by directional arrows 70, to align the direction of polarization of the light wave with the second polarizer 60 having a direction 64 of polarization set at 45 degrees from that of the first linear polarizer 58. In this way, a forward propagating light wave passes through the conventional optical isolator 50 with little attenuation.
A light wave of unknown polarization 74 propagating in the backward direction is first linearly polarized by the second polarizer 60. Since the polarization of light waves traveling in the backward direction is unknown, only light waves traveling in the backward direction with the polarization direction 64 of the second polarizer 60 will pass second polarizer 60 and enter the magneto-optical medium 54. Once propagating in the magneto-optical material 54, the polarization of the backward propagating light wave is rotated by 45 degrees, as shown by directional arrows 72, in the same sense as the rotation of the forward propagating light wave, causing the direction of polarization of the backward propagating light wave exiting the magneto-optical medium 54 to be polarized at 90 degrees with respect to the direction of the first polarizer 58. Therefore, the backward propagated light wave will not pass the first polarizer 58.
With such a conventional optical isolator 50, however, there has been the problem of the need for a bulky magnet for applying a longitudinal magnetic field, stringent modal phase-matching for the TM and TE modes of light waves propagating in the magneto-optical medium and addition of auxiliary components such as polarizers 58, 60. Further, non-uniformities in the longitudinal magnetic field introduce non-uniform polarization rotation across a light wave passing through the magneto-optical medium 54. Unless the light wave dimension is made equal to or smaller than the cross-section of uniform Faraday rotation, these non-uniformities limit the extinction ratio obtainable by the conventional optical isolator.
There exists a need for an optical isolator which eliminates the need for a bulky magnet for applying a longitudinal magnetic field, modal phase-matching and the need for polarizers.